This invention relates to the production of high purity nitrogen, and more particularly to the removal of carbon monoxide from a high purity nitrogen stream by cryogenic adsorption.
Many applications requiring the use of an inert gas, such as nitrogen require that the nitrogen be substantially free of impurities such as carbon monoxide. This is particularly important in the manufacture of computer chips where even very small amounts of impurities can lead to substantial reduction in chip yield. When it is desired to produce high purity nitrogen by cryogenic fractional distillation of air the presence of high boiling, generally noncondensable impurities, such as hydrocarbons, in the air presents no problems, since some of the high boiling impurities can be removed from the feed stream by near ambient temperature adsorption prior to introduction of the air into the distillation system. Hydrocarbons and other high boiling impurities not removed by adsorption leave the distillation system with the oxygen-rich stream. However, if carbon monoxide is present in the air feed as an impurity and it is not removed from the feed stream prior to entry of the feed stream into the distillation system, it will end up in the nitrogen-enriched stream, because its boiling point is very close to that of nitrogen. Conventional removal of carbon monoxide from nitrogen streams requires the use of special techniques, such as high temperature chemisorption or oxidation to carbon dioxide.
Oxidative removal of carbon monoxide requires reaction at temperatures up to 250.degree. C. using platinum-palladium catalysts. Carbon monoixide removal by chemisorption involves the use of copper or nickel based getters/adsorbents at temperatures between 30.degree. and 200.degree. C. Each of these processes significantly adds to the overall cost of high purity nitrogen production.
U.S. Pat. No. 4,746,332 discloses the adsorption of oxygen from a nitrogen stream at cryogenic temperatures using sodium-exchanged A zeolite.
Since most high purity nitrogen is produced by cryogenic distillation, removal of carbon monooxide at cryogenic temperatures would be highly desirable since such a step can be easily integrated into existing processes. The present invention provides such a method.